The present invention relates to a heat exchanger that is adapted for use as an evaporator, a condenser or the like in car air-conditioners, room air-conditioners or the like, and more particularly relates to a heat exchanger comprising heat exchanging tubes which are bent at their intermediate portions between opposite ends.
The heat exchangers recently used in the car air-conditioners are of the so-called multi-flow or parallel flow type. Flat and straight tubes are arranged at regular intervals and in parallel with and spaced apart from each other a predetermined distance in the direction of their thickness. Both the opposite ends of each tube are connected to a pair of hollow headers, in fluid communication therewith.
In the heat exchanger of this type, heat exchange occurs between a medium flowing through the tubes and air streams flowing through air paths each defined between the adjacent tubes. A certain improved heat exchanger is proposed for example in the Japanese Unexamined Patent Publication No. 63-282490. This type is improved in the efficiency of heat exchange and in the drainage of condensed dew, and is of a decreased dimension to fit in a narrower space. Each flat tube in the heat exchanger of this type has, intermediate its opposite ends, a middle portion bent in the direction of its width.
It has however been difficult to protect the tube's internal flow path from collapse when bending it in the width direction.
The U.S. Pat. Nos. 5,279,360 and 5,341,870 propose a method of resolving this problem, in which a plurality of grooves are previously formed along lateral and opposite edges of a tube so that it can easily be bent at its middle region.
There are still problems in those proposals in that the previous forming of many grooves causes much labor and limit the portion where each tube can be bent. In addition, cross-sectional area of the bent portion is so decreased that a pressure loss of the coolant noticeably increases, due to the previously formed groves.
Instead of such a simple bending of tubes in the direction of their width, a `twisting` of tubes is proposed in another Japanese Unexamined Patent Publication No. 4-187990. According to this proposal, a middle portion located intermediate opposite straight sections of each tube is twisted and bent such that the right side surface of one section becomes the left side surface in the other.
This proposal may be effective to avoid the collapse and constriction of the bent portions. It is however necessary to prepare the bent tubes, before connecting them to the headers, to thereby render somewhat intricate the manufacture and assembly of a heat exchanger. In a case wherein the interior of each flat tube is divided into parallel unit paths, a windward one of them in one straight section will continue to a windward unit path in the other, relative to air stream flowing through a space present between the adjacent straight sections. Consequently, heat exchange efficiency will vary among unit paths in each tube, thus impairing the overall efficiency of heat exchange.